It is called a "Hand Rifle" in hunting. The 500-grain .45-70 round it fires has a muzzle velocity of 1500 feet per second and has incredible power for pistol - it can eliminate guards in one shot, regardless of hit location. But the gun's power comes at a price, due to heavy recoil and a single-bullet capacity.
Behind the scenes
This weapon cannot be acquired in normal gameplay and must be unlocked through an emblem or a code.